A plurality of different brake system embodiments are known. An overview of brake systems is shown, for example, in Bosch Kraftfahrtechnische Taschenbuch, 21st edition, pp. 594-643 (Bosch Automotive Handbook, 3.sup.rd edition, pp. 594-643). Such brake systems can be designed as hydraulic, pneumatic, or electromechanical brake systems. Electrical actuation of such brake systems may consist, for example, of actuating the valves affecting the braking pressure in the wheel brakes hydraulically or pneumatically. In the case of an electromechanical brake system, the wheel brakes are actuated electromechanically. For brake systems of motor vehicles of a certain size, the law mandates that, in addition to the service brakes, with which the driver can reduce the vehicle speed during normal operation, an auxiliary brake system and a parking brake system be provided. The auxiliary brake system allows the driver to stepwise reduce the speed or the acceleration of the vehicle or bring the vehicle to a stop in the event of a failure of the service brake system. The parking brake system ensures that the vehicle can be held at a standstill on an inclined roadway by mechanical means, in particular in the absence of the driver.
An electromechanical brake system is described, for example, in International Publication No. WO-A 94/24453, where a wheel brake in which the brake application force is generated by an electric motor. It is particularly important in such electromecanically actuated wheel brakes, as well as generally in the other electrically controlled wheel brakes, to ensure the reliable operation of the brake system, especially reliability of the voltage supply provided by the batteries. In particular, the operating reliability of the brake system must be guaranteed even if sufficient braking action can no longer be applied due to the insufficient amount of electric energy stored.
German Patent Application No. 35 02 100 describes a pneumatic brake system for motor vehicles, where two energy storage devices are provided for supplying power to the brake system; depending on the charge status of one energy storage device, the system allows a switchover to the other storage device. An excessively low charge status can be reported to the driver of the vehicle visually and/or acoustically.
German Patent Application No. 39 36 638 describes an electric supply in a motor vehicle, where, depending on the charge status of the vehicle battery, those loads that are not essential for safe vehicle operation can be shut off.
German Patent Application No. 19548560.2 describes a brake system with electrically actuated wheel brakes, which includes a parking brake that can be actuated by the driver. At vehicle start, a test sequence is initiated to test the functionality of the brake system; during this sequence, at least one wheel brake is supplied with a control signal that corresponds to a predefined minimum braking effect. The parking brake is not released if the predefined minimum braking effect on at least one wheel is not achieved. The charge status of the battery or batteries is not measured.
The object of the present invention is to prevent safety-critical situations in electrically controlled brake systems.